The present invention pertains generally to circuit interrupters employing sulfur hexafluoride (SF.sub.6) in the liquid phase as an arc quenching medium, and pertains particularly to such interrupters in which energy derived from the arc is used to provide at least part of the power for the opening stroke of the interrupter.
Power circuit interrupters using SF.sub.6 as the interrupting medium have been continuously developed and improved over the last two decades. The basic concept of such interrupters is to create a pressure differential in the SF6 by means of a De Laval nozzle to establish the flow of gas from an upstream region to a downstream region along a path intersecting the electric arc that occurs between the contacts when the interrupter is opened. The arc is usually oriented generally parallel to the axis along which the moving contact moves during the opening stroke of the interrupter, and the nozzle is designed and positioned to cause the flow to be approximately axial to the arc.
It is generally desirable in such interrupters to make the pressure differential across the nozzle as great as possible during interruption. By doing so, two things are accomplished. First, the gas flow velocity increases until a sonic velocity is reached. Second, the rate of mass flow through the nozzle increases proportionally to the increase in pressure differential. It has been observed that these effects improve the interrupter's capability of recovering to higher rates of rise of recovery voltage and of interrupting larger fault currents.
The pressure differential across the nozzle has conventionally been obtained either by storing the SF.sub.6 in the gaseous phase at a very high pressure and opening a blast valve to admit it into the arc region through the nozzle, or by the use of a piston coupled to a moving member, usually the moving contact. In the latter arrangement, known as a puffer type circuit breaker, as the moving contact is opened, the piston compresses the gas and forces it through the nozzle. With SF.sub.6 in the gaseous phase, however, there are certain limitations on the maximum pressure differentials that can be obtained. If the gas is precompressed and stored at a high pressure, the low dew point of SF.sub.6 presents a problem, as this substance will liquefy at a pressure of 275 psi at a temperature of approximately 20.degree. C. The maximum operating pressure is thus limited to a figure below 300 psi if the SF.sub.6 is to be maintained in the gaseous phase. In puffer type circuit breakers, on the other hand, the maximum obtainable pressure is limited primarily by the operating mechanism energy requirements, which increase rapidly as a function of the maximum pressure produced. In these circuit breakers, a considerable amount of power is required, since the work of compressing the gas must be performed in a short period of time.
To overcome these limitations encountered when using gaseous SF.sub.6 at high pressure, interrupters have been developed in which SF.sub.6 is used in the liquid phase. Two such arrangements are disclosed, respectively, in U.S. Pat. 4,268,733, issued to the present inventor on May 19, 1981, for A LIQUID SF6 PUFFER TYPE CIRCUIT INTERRUPTER (C-1812), and 4,278,860, issued in Jiing-Liang Wu on July 14, 1981, for an ARC DRIVEN SINGLE PRESSURE TYPE CIRCUIT BREAKER (C-1901). Both of these patents are assigned to the assignee of the present application. In each of the interrupters disclosed in these patents, two pistons that are connected to each other and held a spaced distance apart by insulative tie rods are used to force the liquid SF.sub.6 through a nozzle the quench the arc. The insulative tie rods form an insulative path located electrically in parallel with the insulative tube used as a container for the liquid SF.sub.6. The presence of two parallel insulating paths increases the risk of dielectric failures.
Another problem that has been encountered with previous liquid SF.sub.6 interrupters is that if the interrupted current is relatively small, the energy absorbed by the SF.sub.6 from the arc is too small to cause a sufficiently great increase in pressure, the SF.sub.6 pressure to compensate fully for the increase in the volume occupied by the SF.sub.6 (this volume increase is a result of the fact that the downstream piston is designed to present a larger effective area to the SF.sub.6 than the upstream piston). The drop in pressure may be sufficient to lower the dielectric withstand capability of the SF.sub.6 significantly. This problem can be overcome by designing a circuit breaker for small currents so that its piston assembly has only a short travel, but since a large travel is necessary to permit interruption of large currents, this expedient requires the production of several sizes of circuit breakers for use with currents of different magnitudes.